1. Field of the Invention
The present invention relates to a voltage-controlled capacitive element, in which the capacitance can be controlled by an applied voltage, and a semiconductor integrated circuit (IC) including the same. In particular, the present invention relates to a voltage-controlled capacitive element preferably incorporated into an oscillation circuit (hereinafter referred to as a voltage-controlled oscillator (VCO)), which is used for an electronic apparatus or the like and whose oscillation frequency can be controlled by an applied voltage.
2. Description of the Related Art
MOS (metal oxide semiconductor) type varactor elements have been used as voltage-controlled capacitive elements in semiconductor ICs (for example, see Japanese Patent No. 2951128). A MOS type varactor element is used, for example, for controlling an oscillation frequency of an LC-VCO.
FIG. 1 is a cross-sectional view showing a conventional MOS type varactor element. As shown in FIG. 1, in the varactor element 101, an N well NW101 is disposed in the upper surface of a P type substrate PSub. A gate insulating film 102 is disposed on the N well NW101, and a gate electrode 103, which is formed of poly silicon (polycrystalline silicon) for example, or the like is disposed on the gate insulating film 102. Also, n+ diffusion regions N101 and N102 are placed in two areas in the surface of the N well NW101 sandwiching the gate electrode 103 viewed in the direction vertical to the upper surface of the P type substrate PSub. In the surface of the N well NW101, the region between the n+ diffusion regions N101 and N102 serves as a channel region 104. Further, a p+ diffusion region P101 is placed, in the upper surface of the P type substrate PSub, in part of an area where the N well. NW101 is not disposed.
The n+ diffusion regions N101 and N102 are connected to a well terminal Vb, the gate electrode 103 is connected to a gate terminal Vg, and the p+ diffusion region P101 is connected to a ground potential wiring GND. In FIG. 1, the gate insulating film 102 is disposed only directly under the gate electrode 103, but the gate insulating film 102 may be disposed over the entire upper surface of the P type substrate PSub except an area which contacts (not shown) connected to diffusion regions are disposed. In this varactor element 101, capacitance is generated between the gate electrode 103 and the N well NW101.
In the conventional varactor element 101, a ground potential is applied to the p+ diffusion region P101 through the ground potential wiring GND, so that the P type substrate PSub is at the ground potential. Also, by changing a voltage applied between the gate terminal Vg and the well terminal Vb (hereinafter referred to as voltage between terminals Vgb (=Vg−Vb), the capacitance between the gate electrode 103 and the N well NW101 can be changed. FIG. 2 is a graph showing the voltage dependence of the capacitance in the varactor element 101, in which the horizontal axis indicates the voltage between terminals (Vgb) and the vertical axis indicates the capacitance between the gate terminal Vg and the well terminal Vb.
As shown in FIGS. 1 and 2, by setting the voltage between terminals Vgb at an adequately high value V2, electrons accumulate in the channel region 104 of the N well NW101, so that the varactor element 101 is brought into an accumulation state. As a result, the capacitance of the varactor element 101 reaches a maximum, which is substantially equal to the capacitance of the gate insulating film 102. By decreasing the voltage between terminals Vgb from this state, a depletion layer is generated in the channel region 104 of the N well NW101. As the depletion layer expands, the capacitance of the varactor element 101 decreases along a solid line 53. Then, when the voltage between terminals Vgb reaches an adequately low value V1, expansion of the depletion layer becomes saturated. Accordingly, the capacitance reaches a minimum and does not decrease any more.
However, the above-described prior art has the following problems. By decreasing the voltage between terminals from V2 to V1, the capacitance of the varactor element 101 decreases along the solid line 53, as indicated by an arrow 51. At this time, if the voltage between terminals is instantly changed, the capacitance is also changed instantly. After that, however, even if the voltage between terminals is kept constant at V1, the capacitance gradually increases as indicated by an arrow 52. That is, the capacitance increases by several % to about 10% over several seconds to several minutes, and finally reaches a thermal equilibrium state as indicated by a broken line 54. In this way, even if the voltage between terminals is instantly changed, time is required until the capacitance reaches the thermal equilibrium state indicated by the broken line 54. That is, the capacitance does not quickly follow change in the voltage between terminals. Therefore, when this varactor element is incorporated into a VCO, change in the oscillation frequency thereof is delayed to change in the control voltage, that is, the oscillation frequency does not quickly follow change in the control voltage.